Digital recording systems such as computer disk drives, audio recording/playback (DAT) systems, and video recording/playback (DVD) systems are well known. The majority of these systems use either magnetic recording or magneto-optical recording to store and retrieve data from the storage medium.
Magnetic recording (MR) systems use a magnetic medium (disk or tape) to store data. The magnetic medium contains surface ferro-magnetic particles, each having a magnetic polarity. During recording, the ferro-magnetic particles are exposed to a locally applied magnetic field. The particles become magnetized and the direction of each particle's polarity is used to represent a segment of the recorded signal. During playback, the medium is passed by a playback head which senses the direction of each particle, thereby reconstructing the originally stored data.
Magneto-optical (MO) recording is another system used to store to and retrieve data, common examples being audio and video CD systems. MO systems operate on substantially the same principle as MR systems, both using the direction of ferro-magnetic particles within a magnetic medium (disk or tape) to represent stored information. The MO medium is different from most MR media in that the ferro-magnetic particles within the medium are vertically oriented. MO systems also employ lasers to record and read data from the medium. Data is recorded onto the MO medium by laser heating the MO medium to its curie temperature point. Once the MO medium reaches it curie temperature point, the ferro-magnetic particles within the medium exhibit low coercivity and can be easily re-oriented in another direction when exposed to a magnetic field. A locally applied magnetic field orients particles in the desired direction, the direction corresponding to the data to be recorded. Once the illuminated area cools, the particles exhibit high coercivity and retain their direction even in the presence of strong magnetic fields.
Reading data from the MO medium is accomplished by again illuminating the ferro-magnetic particles with a laser, except at a lower power to avoid heating the medium. The property of the MO medium is such that the embedded particles shifts the polarization of the illuminating light. Surfaces of the MO medium are passed by the illuminating laser and the stored data therein causes a polarization shift in the reflected beam known as the Kerr effect. A detector seizes the changes in polarization and reconstructs the stored data.
The MO medium has greater storage density and retains data more reliably compared to the MR medium. Because the MO medium uses vertically oriented particles, MO media has a recording density typically 10 to 1,000 times greater than that of the MR medium. In addition, because MO systems use a laser to read data from the MO medium as opposed to a playback head, MO media lasts significantly longer than MR media (15-40 years versus 3 years). Further, since MO media is resistant to external magnetic fields at room temperature, data storage is more reliable using the MO "system compared to the MR systems.
The MO media also has a very unique beneficial side effect in that it is an amorphous film. It does not have the crystalline metallic structure of MR film which means that it has very, very fine grains in it which allows the magnetization boundary between an opposite polarity of recording saturation level to be relatively clean and noise-free, so much so that the noise does not increase as the FCI, or flux changes per inch, or flux density increases. That is in opposition to the case with conventional metallic film media where transition noise or zigzag noise as it's commonly called, increases as the FCI increases, or flux density increases.
While MO media provides higher storage density and better reliability than MR media, MR systems are still widely used. This is partly attributable to the relatively slow data rate or the speed at which MO systems can record or retrieve data from the medium. Specifically, MR systems are able to handle more data per unit time, i.e. operate with a higher bandwidth efficiency compared to MO systems.
The difference in the system's bandwidth efficiency is primarily attributable to how the two systems communicate data to and from the storage medium. MO systems conventionally operate using standard saturation or binary level recording.
FIG. 1A shows the block diagram of a conventional MO system 100. The MO system consists of a digital recording channel 110 for receiving an input bit stream 101 and generating a recording signal therefrom, a MO medium 120 for storing the recording signal, and a digital playback channel 130 for reading the recording signal and generating an output bit stream 102. The recording channel 110 includes a binary encoder 111, a writing laser 113, and a magneto-optical recording head 114.
The MO system operates using standard two-level saturation recording technique whereby each received bit in the input bit stream 101 is encoded using a binary encoder 111. The resultant encoded waveform 112 is recorded onto the MO medium, bit by bit, by saturating the magnetic medium 120 to record a 1-bit, or by applying no magnetization to record a zero bit. Playback occurs bit by bit in the reverse order, using a playback head 131, a reading laser 132, an optical reader 134 and a binary decoder 136. Because the recording and playback-signals are digital, the recording and playback channels are not required to be highly linear.